The present invention relates to an optical filter comprising a transparent support and a filter layer. Further, the invention also relates to an anti-reflection film covering a display surface of a display device such as a liquid crystal display device (LCD), a plasma display panel (PDP), an electroluminescence display (ELD), a cathode-ray tube (CRT), a fluorescent indicator tube or a field emission display, to prevent reflection or to improve the color reproducibility of the display.
A display device such as a liquid crystal display device (LCD), a plasma display panel (PDP), an electroluminescence display (ELD), a cathode-ray tube (CRT), a fluorescent indicator tube or a field emission display displays a color image with a combination of the three primary colors (i.e., red, blue, green). However, it is very difficult (substantially impossible) to use the ideal three primary colors. For example, the plasma display panel uses phosphors of the three primary colors, which emit light containing an unnecessary component (in the wavelength region of 560 to 620 nm). Therefore, it has been proposed to correct the color balance of the displayed image by an optical filter absorbing the unnecessary component. The optical filter for the color correction is described in Japanese Patent Provisional Publication Nos. 58(1983)-153904, 60(1985)-118748, 60(1985)-18749, 61(1986)-188501, 3(1991)-231988, 5(1993)-203804, 5(1993)-205643, 7(1995)-307133, 9(1997)-145918, 9(1997)-306366 and 10(1998)-26704.
The display device needs prevention of reflection as well as the color correction. On the screen of the display device, the surrounding scene is often reflected to impair the contrast of the displayed image. Various anti-reflection films have been proposed to solve the problem of reflection. The known anti-reflection layers are categorized into two types, namely evaporating (and depositing) layers and coating layers. The evaporating layers are superior to the coating layers in view of optical characteristics, but the coating layers are easily formed compared with the evaporating layers.
The evaporating layers have been used as anti-reflection films for lenses of glasses or cameras. The layers are generally formed by a vacuum deposition process, a spattering method, an ion plating method, a CVD method or a PVD method.
The coating layers can be formed by coating a dispersion of fine particles and a binder. The coating layers are described in Japanese Patent Provisional Publication Nos. 59(1984)-49501, 59(1984)-50401, 60(1985)-59250 and 7(1995)-48527.
The anti-reflection layers can be introduced into the optical filters. The optical filters having the anti-reflection layers are disclosed in Japanese Patent Provisional Publication Nos. 61(1986)-188501, 5(1993)-205643, 9(1996)-145918, 9(1996)-306366 and 10(1997)-26704. The optical filter described in 61(1986)-188501, 5(1993)-205643, 9(1996)-145918 or 9(1996)-306366 has a transparent support containing a dye or a pigment so that the support functions as an optical filter. Further, the optical filter described in 10(1997)-26704 comprises a colored hard coating (surface hardening) layer provided between a support and an anti-reflection layer, so that the hard coating layer functions as an optical filter.
A colored transparent support or a colored hard coating layer can function as an optical filter. However, it is difficult to incorporate a dye or pigment into the support or the hard coating layer.
The transparent support is made of glass or plastics (usually, plastics). Therefore, the dye or pigment contained in the support must have enough heat resistance to a high temperature in the production process of the support.
The hard coating layer generally comprises a cross-linked polymer. In forming the layer, the polymer is cross-linked after coating a polymer solution. The dye or pigment added in the solution often fades at the cross-linking reaction.
Although there are some dyes which can be added to the support or the hard coating layer, those dyes have too board peaks (i.e., too wide half widths) to give color correction adequate for display devices.
Many dyes have been researched in the field of recording material such as material for silver halide photography or heat sensitive paper. These dyes have various absorption spectra, and have sharp absorption peaks (narrow half widths). However, they have been developed to be contained in a layer of photographic material. If the dyes are incorporated into the support or the hard coating layer, they usually have problems of fading.
The applicant has tried to add the dyes not to the support or the hard coating layer (which restricts the dyes or pigments), but in a polymer layer. The polymer layer can be formed under moderate conditions. Many dyes can be contained in the polymer layer, which functions as an optical filter. However, the polymer layer does not protect the dyes, compared with the support and the hard coating layer. Therefore, the dyes to be contained in the polymer layer are preferably improved in durability (particularly, light resistance). The dyes should be improved without disturbing the function (color correction) of the dye. They should also be improved without absorbing light for an image emitted from the phosphors. Further, they should be improved without coloring the image display device when the image is not displayed.
An object of the present invention is to provide an optical filter having a function of appropriately correcting color.
Another object of the invention is to provide an anti-reflection film having a function of appropriately correcting color as well as a function of preventing reflection.
The present invention provides an optical filter which comprises a transparent support and a filter layer containing a dye and a binder polymer, wherein the dye is in a non-aggregated form having an absorption maximum with a half width of not more than 100 nm within the wavelength region of 560 to 620 nm, and wherein the transparent support, the filter layer or an optional layer contains an o-substituted phenol represented by the formula (I), (II) or (III) as a ultraviolet absorbing agent: 
which the benzene rings a and b may have a substituent group; 
in which Ar1 is an aryl group or an. aromatic heterocyclic group; xe2x80x94Lxe2x80x94 is a single bond or xe2x80x94Oxe2x80x94; and the benzene ring c may have a substituent group; 
in which the benzene ring d and the triazine ring e may have a substituent group; and the benzene ring d may be condensed with another aromatic ring or a heterocyclic ring.
The invention also provides a plasma display panel having a display surface covered with an optical filter which comprises a transparent support and a filter layer containing a dye and a binder polymer, wherein the dye is in a non-aggregated form having an absorption maximum with a half width of not more than 100 nm within the wavelength region of 560 to 620 nm, and wherein the transparent support, the filter layer or an optional layer contains an o-substituted phenol represented by the formula (I), (II) or (III) as a ultraviolet absorbing agent.
The absorption maximum of the optical filter can easily be adjusted, and a sharp absorption peak (a narrow half width) appropriate for an image display device can be given by the dye of the invention. However, the dye contained in a filter layer is unstable to an ultraviolet ray.
According to the study of the applicant, the compound represented by the formula (I), (II) or (III) gives ultraviolet resistance (durability against ultraviolet ray) to the dye. Thus, the dye in a non-aggregated form having an absorption maximum with a narrow half width is now improved in durability by using the o-substituted phenol represented by the formula (I), (II) or (III) without disturbing the function (color correction) of the dye, without absorbing light for an image emitted from the phosphors and without coloring the image display device when the image is not displayed.
A known ultraviolet absorbing agent usually has a problem of durability of the agent itself to an ultraviolet ray. In an optical filter or an anti-reflection film, which is continually exposed to strong light containing ultraviolet ray, the ultraviolet absorbing agent should be stable to an ultraviolet ray to continue the ultraviolet absorbing function. The o-substituted phenol represented by the formula (I), (II) or (III) is stable to the ultraviolet ray. Therefore, the ultraviolet absorbing agent can protect the non-aggregated dye from the ultraviolet ray for a long term.
Thus, the optical filter and the anti-reflection film of the invention have a function of adequate color correction according to the aimed image display device.